There are many test applications that require an optical signal to be modulated at a radio or microwave frequency. Such applications include receiver testing, optical-based microwave generation, testing of microwave components, direct optical injection of source signals for high frequency testing (photo-detector on chip) and fiber delivery of microwave signals for remote antennae testing. Recently, optical heterodyne techniques have been developed to modulate optical signals at radio or microwave frequencies. Such techniques typically combine signals from two optical sources, such as lasers, characterized by different frequencies. Due to the superposition principle, the resulting combined signal is characterized by a heterodyne frequency equal to the difference between the two individual laser frequencies. This technique is less expensive than direct modulation using a RF or microwave modulator.
FIG. 1 shows a block diagram of a prior art optical heterodyne calibration system 100 used to calibrate a device, known as the device under test (DUT) 101. The DUT produces an electrical signal in response to a modulated optical signal that it receives from a beat note generator 102. In the beat note generator 102 two lasers 103, 104 produce optical signals characterized by slightly different frequencies. A 50:50 coupler 105 combines the signals from the lasers 103, 104 to produce a heterodyne modulated optical signal. One portion of the heterodyne modulated optical signal is coupled to an input of the DUT 101 while another portion is coupled to optical-to-electronic (OE) converter 106. The OE converter 106 produces an electrical signal characterized by the frequency and phase (shifted 180 with the configuration shown in FIG. 1) of the heterodyne modulated optical signal coupled to the DUT 101. The electrical signal is then coupled to an electronics unit 108, which may provide control signals to the lasers 102, 104 and/or an optional variable optical attenuator (VOA) 110 for balancing the power output of the lasers. The DUT 101 produces an output electrical signal that may be coupled to a measuring device 112 such as an RF power meter.
Calibrating optical components often requires measurement of amplitude and phase of the output electrical signal from the DUT 101 relative to the heterodyne modulated optical signal applied to the input of the DUT 101. It is therefore desirable to measure both the amplitude and the phase of the heterodyne modulated optical signal from the coupler. Unfortunately, while it is relatively straightforward to measure the amplitude and phase of a heterodyne signal modulated at frequencies below about 10 Gigahertz (GHz); measuring the phase of such a signal at frequencies above about 10 GHz is problematic.
For example, FIG. 2A depicts a block diagram of a first prior art high frequency phase measuring scheme. In this scheme a beat note generator 202 produces a heterodyne modulated optical output that is coupled to the input of a DUT 201 and a reference OE detector 203. Electrical outputs from the DUT 201 and the reference OE detector 203 are coupled to a phase detector 204. The drawback of this scheme is that it is both difficult and expensive to calibrate the reference OE detector. Generally for opto-electronic devices, the higher the frequency the more difficult and expensive the calibration.
FIG. 2B depicts a block diagram of a more standard prior art solution for high frequency phase response measurement. A 0-100 GHz swept oscillator 222 produces both a driving signal and a reference signal. The driving signal drives a calibrated reference electronic-to-optical (EO) converter 224. The reference OE converter 224 produces a modulated optical signal that is coupled to a DUT 226. The output of the DUT 226 and the reference signal from the oscillator 222 are coupled to a phase detector 228. Although such a system is commercially available, the oscillator 222 and the reference EO converter 224 are expensive due to the cost of the high-speed electronics that they contain and the cost of the required calibrations. Systems of the type shown in FIG. 2B can cost approximately $400,000.
Thus, there is a need in the art, for high frequency phase response measurement of opto-electronic devices without incurring the expense associated with high-speed electronics and calibrations of high-speed EO or OE converters.